A chain reaction could easily be sustained in a reactor containing fuel elements and a moderator. In fact, the reaction might occur so quickly that the reactor would explode. In order to prevent such a disaster, the reactor core also contains control rods. Control rods are solid cylinders of metal constructed of some material that has an ability to absorb neutrons. One of the metals most commonly used in the manufacture of control rods is cadmium.
The purpose of control rods is to maintain the ratio of neutrons used up in fission compared to neutrons produced during fission at about 1:1. In such a case, for every one new neutron that is used up in causing a fission reaction, one new neutron becomes available to bring about the next fission reaction.
The problem is that the actual ratio of neutrons produced to neutrons used up in a fission reaction is closer to 2:1 or 3:1. That is, neutrons are produced so rapidly that the chain reaction goes very quickly and is soon out of control. By correctly positioning control rods in the reactor core, however, many of the excess neutrons produced by fission can be removed from the core and the reaction can be kept under control.
The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1:1 ratio of produced to used up neutrons is achieved. At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators.
Science EncyclopediaScience & Philosophy: Nicotinamide adenine dinucleotide phosphate (NADP) to Ockham's razorNuclear Reactor - Theory Of Fission Reactors, Reactor Core, Moderators, Control Rods, Reactor Types, Applications